I am sorry that I can't answer in French but I think I might understand enough of your question to answer.
'I am using LV7 which uses the system clock. It is very easy for someone to modify the time and date. For security, is there a way to prevent this? Link to a server?'
Please ignore if I've missed the point!
a) You can have your application directly query a time server and then set the system time itself. Don't forget to handle the error condition when the timeserver is unavailable 😉 See 'query a time server'.
b) Your application can detect changes in the system clock by reading the current time, adding a known value (i.e. 60 seconds), then wait 60 seconds and compare the current time to the predicted time. This wi
ll allow you to detect and log system time changes greater than 60 seconds while ignoring small changes if the OS makes corrections. See 'catch system time change'.
Hope this is pertinent and helps.
Matt
